1. Field of the Invention
This invention relates generally to a plastic ball grid array (xe2x80x9cPBGAxe2x80x9d) assembly, and more particularly, to an improved PBGA assembly including strengthened corner sections.
2. Description of the Related Art
A PBGA assembly is a package for enclosing semiconductor logic chips. PBGA assemblies are designed to increase the number of paths available for delivering signals to and from the logic chips. For example, conventional semiconductor packages commonly have a single row of pins disposed about their outside edge. Whereas PBGA assemblies employ the larger surface area under the PBGA assembly to connect signals to the internal logic chips, e.g., a plurality of solder balls spaced about the bottom surface area are used for the transfer of signals.
A PBGA assembly generally comprises two parts: a substrate and an overmold. During assembly of the package, the logic chip or die is glued, or otherwise attached, to the top of the substrate. The overmold is then molded on the substrate over the die to form a protective covering.
The substrate is a relatively thin member, commonly having a thickness of about 0.36 to about 0.65 millimeters thick. Further, the substrate can include printed circuit traces, dielectric material, and solder balls. Typically, the substrate is either square or rectangular in shape, and is relatively flexible. Substrates generally range in sizes from about 27 to about 50 millimeters in width and in length. Overall, substrates are relatively fragile devices, prone to damage if not protected.
A typical PBGA assembly 10 is shown in FIGS. 1A and 1B. The assembly 10 includes a mold cap 12 disposed upon a substrate 14. The mold cap 12 includes chamfered side edges 16 and chamfered corners 18. Disposed beneath the substrate 14 are a plurality of solder balls 20 for use in connecting the PBGA assembly 10 to a printed circuit board (not shown), and in transferring signals to and from a logic chip 22 mounted on the substrate 14 and covered by the mold cap 12.
The solder balls 20 disposed along the bottom of the substrate 14 provide the connections between the PBGA assembly 10 and the printed circuit board. Installation of the PBGA assembly 10 onto the printed circuit board is relatively straight forward. The PBGA assembly 10 is heated during installation to a relatively high temperature so that the solder balls 20 melt and the PBGA assembly 10 in effect solders itself to the printed circuit board.
The mold cap 12 comprises a mold compound of sufficient thickness to form a protective covering over the die 22 (typically, about 0.8 millimeters thick). Typically, the mold compound is a thermoset plastic compound, and is used to protect the die 22 from environmental conditions. The mold cap 12 extends over all but the four side edges of the substrate 14, so that the edges of the substrate 14 remain free for clamping during the. molding operation (e.g., to hold the substrate 14 in place proximate the mold during the overmolding process when plastic is injected into the mold), and during subsequent processing, handling and installation. The edges 16 of the mold cap 12 are chamfered to facilitate removal from the mold. The mold cap edges 16 define an acute angle and are designed to ease the separation of the mold cap 12 from the mold. The mold cap 12 also typically includes chamfered corners 18. Thus, the corner sections 19 of prior PBGA assemblies 10 are unsupported, and generally consist of only the substrate 14.
As a result of this construction, the corner sections 19 of prior PBGA assemblies 10 are susceptible to mechanical damage. The flexible corner sections 19 are easily damaged, for example, during processing associated with the assembly, test, and installation of the PBGA assemblies 10, and during normal handling.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.
In one aspect of the present invention, an apparatus for enclosing a logic chip is provided. The apparatus includes a substrate, which has a surface adapted to receive the logic chip mounted thereon. Further, a mold cap is disposed upon the substrate, and the mold cap extends at least partially over the logic chip. The mold cap also includes at least one extension of a preselected size and shape that extends at least partially into a corner section of the substrate.
In another aspect of the present invention, a method is provided for enclosing a logic chip. The method includes mounting the logic chip on a polygon shaped substrate, and forming a mold cap on the substrate extending at least partially over the logic chip and into a corner section of the substrate.